Calendering paper by supercalendering, hard nip calendering or soft nip calendering has been used in the paper industry for many years and the operations of such calenders have been studied and numerous reports written.
In Pulp and Paper Canada, Volume 75, No. 11, November 1974 in a paper by Kerekes and Pye entitled "Newsprint Calendering: An Experimental Comparison of Temperature and Loading Effects", an experimental program is reported wherein nip loading, number of nips and temperature of the rolls were discussed particularly in reaction to hard roll calendering. The conclusions reached indicate that heating the rolls can reduce the number of nips or pressure necessary to obtain a selected density and that higher temperature rolls improve the gloss.
Canadian Pat. No. 1,011,585 issued June 7, 1977 to Anderson is one of the earlier discussions of what appears to be a moisture and temperature gradient calendering system wherein moisture is applied to the sheet and then the sheet is drawn over a hot platen.
The concept of moisture gradient calendering was discussed in British Board Industries Fed., London, 1978, Vol. 2, pages 641-669 in an article entitled "The Effect of Moisture and Moisture Gradients on the Calendering of Paper" by Lyne. In this paper the concept of plastic flow and softening of lignin by moisture and temperature is discussed and the effect of these parameters on the surface of a web under conditions of conventional roll temperature, high roll temperatures and a combination of high roll temperatures and moisture application immediately before the calender nip is examined.
In an article entitled "Temperature-gradient Calendering" by Crotogino published in the Tappi Journal/October 1982, pages 97-101, the effect temperature gradient calendering of paper is described. The surface of the paper web is heated but the heat does not have time to penetrate to the inside of the paper web which remains relatively cool as the web passes through the calender nip. This results in development of improved surface properties without significant compaction of the middle of the sheet thereby to reduce strength loss by the calendering operation.
Dunfield et al, in an article entitled "Gravure Printability of Steam-Treated Machine Calendered Newsprint" published in the Journal of Pulp and Paper Science: Vol. 12, No. 2 March 1986, describes, among other things, the application of steam in a steam shower before the nip of a calender and indicates that improved results can be obtained by moisture gradient calendering even without added heat.
U.S. Pat. Nos. 4,624,744 issued Nov. 25, 1986 and 4,749,445 issued June 7, 1988 to Vreeland describe the effects of temperature gradient calendering using a soft backing roll.
In a paper given at the 1988 Annual Meeting of the Canadian Pulp and Paper Association, Technical Section, Jan. 28-29, 1988, entitled "The Effect of Calender Steam Treatment and Roll Temperature on Newsprint Properties" by Keller, the effect of temperature gradient calendering with steam treatment before the high temperature nip was further examined and it was found that the steam treatment before the high temperature nip produced the highest MD Tensile and CD Tear strength values for a given roughness. Also the use of a high roll temperature increases gloss for a given density. It was further noted that the application of steam slightly reduced the beneficial effect of the high calender temperature on gloss and reported that the addition of steam even when calendering to higher densities did not develop the same gloss as was developed using high temperature rolls only.
In a paper entitled "The Effects of Z-Direction Moisture and Temperature Gradients in the Calendering of Newsprint" by Gratton et al, published in Journal of Pulp and Paper Science, Volume 14, No. 4, July 1988, pages J82-J90, the effect of both temperature and moisture gradient calendering on the surface of paper were examined and it was concluded that the application of moisture to the surface of the paper when using a high temperature calender roll was not significantly different compared with calendering with a roll at the same temperature without added moisture.
A paper entitled "Effects of Moisture and Temperature on Paper Properties with Implications for Hot Calendering", Kalender Seminar Konigsbronn 1988-03-25 by Black, further discusses moisture content and temperature in the calendering nip and concludes that one of the major advantages of hot calendering is rapid solidification on the exit side of the hot nip thereby reducing springback indicating that a high moisture content is only an advantage when sufficient water is evaporated in the nip to achieve a rapid solidification. This paper also suggests that it might be advantageous from a brightness point of view to limit the temperature of the surface to less than about 150.degree. C.
Miscellaneous reports MR 109 by the Pulp and Paper Research Institute of Canada, in a paper entitled "Hard Nip and Soft Nip Calendering of Uncoated Groundwood Papers", March 1987 by Crotogino et al, a review of the established, new emerging calendering techniques is provided and it is stated that steam showers may be used to help reduce bulk and improve the surface properties of a paper.
In the Black paper above discussed, it has been suggested that too high a roll temperature in the calender may be detrimental to the gloss characteristics of the treated paper.
It will be apparent from the above that the concepts of temperature and moisture gradient calendering have been well investigated and the basic conclusion reached by the experts is that temperature gradient or high temperature calendering is beneficial, that moisture gradient calendering is also beneficial but when both moisture and temperature gradient calendering are used, gloss is substantially the same or possibly reduced compared with that obtained if only temperature gradient calendering were being carried out.
In a paper entitled "Calender Steam Showers--An Effective New Way of Hot Calendering" by Hilden et al, Tappi Journal, Vol. 70, July 1987, describes a new design of steam shower for applying steam in the calender stack. This shower is more fully described, it is believed, in the October, 1988 Tappi Journal entitled "Practical Aspects of Calender Steam Showers" by Vyse et al, pages 87-90.
In a more detailed article entitled Calender Steam Showers--A New Effective Means of Hot Calendering by Hilden and Sawley published in Pulp and Paper Canada, Volume 88, No.12 (1987) T452-T455 the steam application of steam to paper in the calenders is described. Similar equipment and processes are described in the Finishing and Converting Conference 1986, TAPPI Proceedings pages 95-100 by Hilden and Sawley and the May 14-16, 1987 Spring Conference of the Canadian Pulp and Paper Association Preprints pages 1-6 inclusive. All of the three publications are recited since there are minor differences in what is disclosed in each publication. For example, in the Pulp and Paper reference, high energy transfer is referred to and a steam iron effect which includes contact between the sheet and the steam applicator is described whereas in the other papers no contact is referred to. In the latest publication, of which applicant is aware and which it is believed relates to the same steam shower device, entitled Controlling Paper Smoothness using Calender Steam Showers by Vyse and Sawley presented in 1989 at the Annual Meeting of the Canadian Pulp and Paper Association, (see pages A205-A209 in the preprints for the meeting), further information is provided on the steam pressures and temperatures used in the system and steam showers are stated to be close to, i.e. close clearance from the sheet (page A206, Column 1, line 6).
In the later publication the steam pressure supplied to the steam shower is defined as high as 8 psig at temperatures of 115-125.degree. C.
In the earlier papers, the highest temperature that could be reached on the paper was 100.degree. C. and this was determined by the temperature of the sheet approaching the steam shower. No absolute value for the sheet temperature is described in the latest paper however the plotted temperature increase appears to reach about 20.degree. in FIG. 3 but is defined as 23.degree. minimum in Table 1.
In all cases the increase in smoothness obtainable when practicing the method of these papers appears to be up to about 10 points (Sheffield smoothness) improvement which is barely significant.
In all of the above devices wherein steam is applied to the web while the steam supplied to the applicator or shower may contain a minor degree of superheat, by the time it is cooled by the applicator itself and reaches the paper, the steam likely is about saturated, i.e. a temperature of probably no more than 100.degree. C.
One of the problems in utilizing current technology is condensation problem of the steam not contained within the intestees of the webs. Also, the use of saturated steam limits the maximum temperature to which the surface of the sheet may be raised to about 100.degree. C.
The use of superheated steam in paper making is not new. The effect of the application of super heated steam during drying is discussed in a paper entitled "Effect of Superheated Steam Drying on Paper Properties" by David et al, was presented to the Annual Meeting of the Canadian Pulp and Paper Association, Technical Section, Jan. 28-29, 1988, see pages B233-B237 of the preprints. In this paper, superheated steam is applied to a handsheet to dry the handsheet and it was found that some of the physical and optical properties of the resultant dry paper were significantly better than those that would be obtained with normal air drying, however no practical way of drying using superheated steam is suggested.